Within the scope of the topic of CO2, increasingly efficient current converters are needed. Examples include inverters for photovoltaic or automotive applications. High-level blocking, low-loss and rapidly-switching power semiconductors are necessary for this. Apart from active semiconductor switches, such as IGBT's or CoolMOS transistors, free-wheeling diodes are also necessary. Generally, PIN diodes made of silicon are used for high-voltage applications. PIN diodes exhibit low forward voltages and low reverse currents and, consequently, low forward and reverse losses. However, high switching losses, which occur as switching-off losses during the commutation of the current, are disadvantageous.
High-voltage PIN diodes are PN diodes, in which an undoped (intrinsic) and, in practice, mostly lightly doped layer i is situated between p-type and n-type regions. The reverse voltage is chiefly received by lightly doped region i. The space charge region extends mainly in the lightly doped region. The dopant concentration and the thickness of this lightly doped region are determined by the specified breakdown voltage. A high breakdown voltage means a low dopant concentration and high thickness of this lightly doped region. For a 600 V diode, the dopant concentration of the i-layer is approximately 3·1014 cm−3, and the layer thickness is approximately 50 micrometers.
During conducting-state operation at a high current density, high-level injection occurs in PIN diodes. In this context, electrons and holes are injected into the lightly doped region. In the process, the concentration of the injected minority carriers exceeds the dopant concentration of the lightly doped region. This markedly increases the conductivity of the lightly doped region. Consequently, the voltage drop at the lightly doped center region remains small. The forward voltage at high currents remains low. In contrast to that, no increase in the charge carrier density takes place in majority carrier components, such as Schottky diodes. The lightly doped region constitutes a large ohmic resistor, at which a correspondingly high voltage decreases in the forward direction.
During switching-off, the charge carriers (electrons and holes), which are injected in the forward direction into the lightly doped region during the operation of PIN diodes, must first be depleted before the diode can take on reverse voltage. Thus, in response to abrupt commutation of the current, the current initially continues to flow in the reverse direction, until the stored charge carriers are removed or depleted. This current is also referred to as depletion current or reverse recovery current. The magnitude and duration of the depletion current is determined, first and foremost, by the amount of charge carriers stored in the lightly doped region. The more charge carriers present, the higher the depletion current. A higher depletion current means a higher switching-off power loss. By integrating the switching-off current with respect to time, one obtains stored charge Qrr (reverse recovery charge), which is an important variable for describing the switching-off power loss and should be as low as possible. In the case of PIN diodes, switching times and switching losses are high.
Schottky diodes (metal-semiconductor contacts or silicide-semiconductor contacts) provide an improvement in the switching performance. In the case of Schottky diodes, high-level injection does not take place during conducting-state operation. Therefore, the need for sweeping out the minority carriers is eliminated. Schottky diodes switch rapidly and with nearly no loss. However, thick and lightly doped semiconductor layers are again necessary for high reverse voltages, which, in the case of high currents, results in unacceptable, high forward voltages. Therefore, in spite of good switching performance, power Schottky diodes manufactured using silicon technology are not suitable for reverse voltages over approximately 100 V.
A semiconductor component, which is referred to in the following as a cool SED diode, is described in German Patent Application No. DE 197 40 195 C2. In this diode, the introduction of doped, alternatingly positioned p-type and n-type conducting columns under a Schottky contact allows the resistance to be lowered almost as much as desired. If the column width is reduced, the column dopings may be increased. In this context, the doping of the p-type and n-type columns is selected, such that all of the dopant atoms are ionized in response to the application of a reverse voltage. This principle is also referred to as the super-junction principle (SJ). Since a certain minority carrier injection takes place via the p-type doped columns, the ideal switching performance of a pure Schottky diode is not attained, but it is markedly improved over a PIN diode. However, the low forward voltage of the PIN diode is not obtained at high currents. The super-junction principle is described, for example, in the magazine, Japanese Journal of Applied Physics, Vol. 36, pages 6254-6262.
A super-junction Schottky oxide PIN diode is described in German Patent Application No. DE 10 2011 080 258.4. This has a trench structure including parallelly connected Schottky and PIN diodes, in which the Schottky and the PIN regions are galvanically separated and have charge carrier compensation (super-junction structure). The galvanic separation of the Schottky and PIN structure allows high-level injection to take place in the PIN regions. At nearly comparable switching-off losses, the forward voltage is less than in the case of the conventional cool SBD diode.
A cross section of a detail of an example of a super-junction Schottky oxide PIN diode (SJSOP) is illustrated in FIG. 1. A SJSOP is made up of an n+-type substrate 10, on which an n-type epitaxial layer 20 of thickness D_epi and dopant concentration ND is situated. The n-type epitaxial layer 20 includes etched-in trenches 30, which are filled with p-type silicon of dopant concentration NA and are filled with p+-type silicon 40 on the upper side. The width of n-type regions 20 is Wn, and that of the p-type and p+-type regions 30 and 40 is Wp. The dopant concentrations and widths are selected so that all of the regions 20 and 30 are depleted upon application of the full reverse voltage (super-junction principle). This is approximately the case when NA·Wp=ND·Wn=1012 cm−2. Dielectric layers 70, which are preferably SiO2 layers having a thickness D_ox, are situated between the p/p+-type and n-type regions. In this manner, the p-type and n-type regions are not directly connected electrically. On the front side of the chip, n-type regions 20 and p+-type regions 40 are covered by a continuous metallic layer 50, which forms a Schottky contact with n-type regions 20 and an ohmic contact with p+-type regions 40. Metallic layer 50 constitutes the anode contact of the diode. The barrier height of Schottky diode 50-20 may be set by selecting an appropriate metal 50. For example, nickel or NiSi may be used as a metallic layer 50. In some instances, other metallic layers not drawn in may be situated over functional layer 50, in order to render the top surface solderable or bondable. A metallic layer or metal system 60, which constitutes the ohmic contact to heavily doped n+-type substrate 10, is situated on the back side. This layer or layer sequence is usually suitable for assembly by soldering or in another manner. It may be made up of a sequence of Cr, NiV and Ag. Metal system 60 forms the cathode terminal.
One may view the set-up as parallelly connected Schottky and PIN diodes. In this context, metallic contacts 50 form, together with n-type columns 20, Schottky diodes. The PIN structure is formed by the layer sequence made up of p+-type region 40, p-type region 30 and substrate 10 as a p+/p/n+structure.
When the reverse voltage is applied, the p-type and n-type columns become depleted of carriers. The doping may be increased with decreasing width Wp and Wn, at least up to a certain limit that results from the fact that the space charge regions already collide at a low voltage. This reduces the bulk resistance of Schottky diodes 50-20-10 in the forward direction. Therefore, the forward voltages are lower than in the case of a simple Schottky diode, which has a lower degree of doping at the same reverse voltage. In addition, a small current still flows through the PIN diodes in the forward direction. This further reduces the forward voltage. However, upon de-energization, the minority carriers must also be swept out again with detrimental effects on the switching time.